Computing devices are ubiquitous. For example, people often carry smartphones with them at nearly all times and locations. Modern smartphones provide many of the capabilities of a traditional desktop or laptop computing device. People typically use applications running on smartphones for many different purposes, such as for communication, entertainment, shopping, education, and to manage personal or professional affairs. These applications may expose private or confidential information in such a way that the information can be perceived by others in proximity to the user of the computing device.